1. Field of the Invention
The present invention relates to a detecting circuit of a phase-locked loop (PLL) system for use in a television (TV) receiver or the like and particularly to a detecting circuit which reduces buzz produced in a sound signal.
2. Description of the Background Art
The intercarrier system is conventionally used as a sound receiving system in television broadcasting. The intercarrier system is excellent in simplicity of circuit structure, stable operation and the manufacturing cost compared with a separate carrier system. However, the intercarrier system has a disadvantage that a buzz is produced in a sound signal after detection due to an influence of a slope through a filter provided at a preceding stage of a video intermediate frequency (IF) amplifying circuit. A television receiver which reduces generation of the buzz is described in Japanese Patent Laying-Open No. 55-136773. FIG. 2 is a circuit diagram showing the above mentioned television receiver. This television receiver comprises a tuner 1, an input filter 2, a video IF amplifying circuit 3, a correction filter for correcting a slope through the video IF amplifying circuit 3, a tuning circuit 5 and an intercarrier detecting circuit 6. The frequency characteristic of the input filter 2 shown in FIG. 2 is set to an inclination--.alpha. of a slope around a video IF frequency fp. Accordingly, the frequency characteristic of the correction filter 4 is set to an inverse inclination .alpha. so that a predetermined band f of the above mentioned inclination--.alpha. may be flat as shown in (b) of FIG. 3. The tuning circuit 5 amplifies a sound IF frequency fs and its frequency characteristic is set as shown in (c) of FIG. 3. Consequently, the general frequency characteristic for the range reaching the input stage of the intercarrier detecting circuit 6 is as shown in (d) of FIG. 3. The intercarrier detecting circuit 6 detects a sound FM signal of 4.5 MHz as a beat of the video IF frequency fp and the sound IF frequency fs. On that occasion, a sideband of the video IF frequency fp is not influenced by the slope and the sound FM signal is not phase-modulated as can be clearly seen from (d) of FIG. 3. In consequence, even if the sound FM signal is demodulated by an FM demodulating circuit (not shown) in the succeeding stage, buzz is not produced in the sound signal.
However, in the circuit shown in FIG. 2, frequency components f1 to f4 of the sideband of the video IF signal are multiplied by a frequency fs of the sound at the time of detecting the sound FM signal of 4.5 MHz in the intercarrier detecting circuit 6 and as a result interference waves (for example, f1-fs, f2-fs, f3-f5 and f4-fs) causing buzz are generated. In general, the sound FM signal is subjected to amplitude limitation by a limiter and then it is demodulated. The differential phase (DP) characteristic of the sound FM signal is liable to be deteriorated in the multiplier, the limiter and the FM demodulating circuit and if the DP characteristic is deteriorated, the above mentioned interference waves undergo phase modulation. If the phase-modulated interference waves are FM demodulated, phase-modulated components are FM-detected and accordingly buzz is generated in the detected sound signal.
Since the correction filter 4 shown in (b) of FIG. 3 is provided in a signal path in the circuit of FIG. 2, the amplitude of the sound IF signal is considerably limited by the frequency characteristic, causing deterioration of the S/N ratio of the sound signal.